small UI improvements

[Артем Попов <tfwo mail ru>]

Here're some ideas on small beast UI improvements:

* wpdate the main toolbar:
>>  use full-sized icons
>>  add save/undo/redo (common actions) to the toolbar

* update mixer editor to use horizontal toolbar (for consistent look
with other parts: routing/song editor)

* rename Parameters in CSynth/Song tab to properties and add icon
(stock) to it as well.

I'm ready to take on this tasks, if they seem ok. --Artem